1. Field of the Invention
This invention relates generally to interventional medical devices, and more particularly concerns an electrically conductive, composite resistive heating catheter shaft having variable stiffness for enhanced performance of the composite shaft when used with or without a guide catheter, or as a stand-alone flow directed device for use in the vascular system as part of a therapeutic system or for delivery of medical devices.
2. Description of Related Art
Conventional minimally invasive catheter based therapies typically require guide wires that are one to two meters long extending through a longitudinal lumen in the catheter, and that are torqueable and pushable at the proximal end, yet soft and flexible at the distal end. Many such guidewires are made of stainless steel or the like, and are ground to tapers which provide the desired bending properties along the guidewire. Recently, numerous minimally invasive sensing and actuation procedures have been developed which utilize an optical fiber to deliver optical light or power to the distal tip of the optical fiber. For example, optical fiber based technology can be used for treatments such as xe2x80x9cthrombolyzingxe2x80x9d blood or cutting tissue by use of high energy light delivered through the end of the optical fibers, and for the delivery of therapeutic agents, such as timed release agents or embolics. However, conventional optical fiber technology has not been easily adaptable to such applications, particularly when the optical fiber must also act as a guidewire, either within a catheter or as a stand-alone device, since optical fibers, when used alone, are not very torqueable, pushable or resilient when compared to guide wires made from a variety of other, more rigid, materials. Also, small diameter optical fibers are quite xe2x80x9cfloppyxe2x80x9d and in very small diameters are likely to kink, while larger diameter fibers which perform better in that regard can be too stiff to maneuver through sharp bends, and the use of optical fibers as guidewires or pushers within catheters can thus be difficult and quite technique sensitive.
A variable stiffness catheter having a longitudinal lumen is known that is composed of a relatively flexible outer coaxial tube and at least two tandemly disposed inner coaxial tube segments, the tube segments varying in stiffness, with the stiffest being located at the proximal end of the catheter and the least stiff ending proximal of the distal end of the catheter, thus providing the catheter with a minimum of two regions of different stiffness and flexibility. In order to reinforce a wide variety of catheters incorporating longitudinal lumens for interventional therapies, catheters in the prior art have used reinforcements to the exterior of the catheter, including additional strengthening layers and the like to alter the bending characteristics of the catheter. However, such a catheter structure is typically only capable of being used with a guidewire.
It would be desirable to provide an electrically conductive catheter shaft including one or more electrically conductive members with variable stiffness to be more pushable at the proximal end and more trackable at the distal end, and to make the use of such electrically conductive members in catheter-based therapies utilizing resistive heating more straightforward and less technique sensitive. The present invention addresses these and numerous other needs.
Briefly, and in general terms, the present invention in its broadest aspect provides for a variable stiffness electrically conductive composite, resistive heating catheter shaft, with a variable stiffness jacket encapsulating the shaft to make the use of such a shaft in catheter based therapies more predictable, straight forward, and less technique sensitive. Typically, such a shaft can be an electrically conductive member or the like which by itself has physical characteristics that are undesirable for guidewires or pusher devices. By use of the invention, a variable stiffness shaft can be made which is more pushable at the proximal end and more trackable at the distal end, with the capability to provide a wide range of predictable variations in stiffness and other structural parameters over the length of the shaft. It has been found that it is often the case that a catheter shaft such as an optical fiber or ultrasonic conductor member is made of a material that has undesirable characteristics for guidewires, since they are generally of a less resilient and strong material than those typically chosen for guidewires. The invention overcomes these limitations by providing for means to selectively strengthen the catheter shaft by low profile overlays of materials to create a composite shaft. A variable stiffness electrically conductive catheter shaft constructed according to the invention can be used in conjunction with a guide catheter or as a pusher catheter.
By using the construction according to the invention, coating or heat shrinking PTFE on the outside diameter of the one or more electrically conductive members will improve tracking of the device, and heat shrinking layers of PTFE, braid or coil imbedded in a polymer layer, or other polymers in telescoping fashion from proximal to distal end will yield a shaft with a stiffer, more manageable, proximal end and a softer, more maneuverable, distal tip.
The invention accordingly provides in a presently preferred embodiment for a variable stiffness electrically conductive composite resistive heating catheter shaft for placement within the vascular system, and the invention is particularly adaptable for use within a tortuous, small diameter vessel such as those found in the vasculature of the brain. The variable stiffness electrically conductive composite resistive heating catheter shaft comprises at least one electrically conductive member having a proximal end and a distal end, and at least one coaxial layer of a polymer, metal, or both for providing a desired additional stiffness extending over the at least one electrically conductive member, to thereby provide desired variations in stiffness along the length of the shaft. In one presently preferred embodiment, the variable stiffness electrically conductive catheter shaft comprises a plurality of coaxial layers of heat shrink polymer encapsulating the at least one electrically conductive member, the coaxial layers extending from the proximal end of the at least one electrically conductive member toward the distal end, the plurality of coaxial layers having different lengths to provide the electrically conductive catheter shaft with varying stiffness over the length of the electrically conductive catheter shaft. The plurality of coaxial layers can be arranged in successive progressively shorter coaxial layers, and can be formed of heat shrink polymeric material, such as polytetra fluoro ethylene (PTFE) polyethylene terephthalate (PET), polyether ethel ketone (PEEK), poly phenylene sulfide (PPS), or any of a variety of other polymers which can be fabricated into a structure and necked or shrunk over a shaft. A layer of braid or coil may also be embedded in the polymer to increase the stiffness of the composite shaft in certain areas.
While the invention can effectively use tubes which are placed over the exterior of the electrically conductive catheter shaft and then heat shrunk or bonded by adhesive to the at least one electrically conductive member, it is also contemplated that the shaft can be reinforced by other longitudinally extending additional structures with varying cross sections for certain specific applications.
In a presently preferred embodiment, the variable stiffness electrically conductive composite resistive heating catheter shaft comprises a first coaxial layer of a heat shrink polymer extending essentially the entire length of the at least one electrically conductive member, from the proximal end to the distal end; a second layer of a coaxial layer of a heat shrink polymer, of the same or different material as the first coaxial layer, extending over the first coaxial layer from the proximal end of the at least one electrically conductive member to a distal position spaced proximally from the distal end of the at least one electrically conductive member; and a third coaxial layer of a heat shrink polymer, of the same or different material as the first and second coaxial layers, extending over the second coaxial layer from the proximal end of the at least one electrically conductive member to a distal position spaced proximally from the distal position of the second coaxial layer.
The successive coaxial layers of heat shrink tubing are placed on the at least one electrically conductive member extending from the proximal end and ending at different distances from the distal tip of the at least one electrically conductive member. Heat can be applied to the successively applied coaxial layers of tubing, resulting in shrinkage of the tubing to encapsulate the at least one electrically conductive member, creating a tapered shaft having a variable diameter without edges in the outer surface of the shaft. The tapered structure formed by the successive layers of tubing allows the proximal part of the composite shaft to be relatively stiff, and the distal tip to be flexible and soft. A variety of other techniques can be used within the scope of the invention to accomplish the variable stiffness of the electrically conductive catheter shaft. Such techniques include, but are not limited to, the use of a tapered extrusion for the jacket, butt welding of segments of material with a different stiffness from one another to form the jacket, and use of an adhesively bonded hypo tube of stainless steel or the like as a jacket, possibly with a ground taper to the hypo tube, and braid or coil reinforcing embedded in a polymeric layer.
In another aspect of the invention, the variable stiffness electrically conductive catheter shaft further comprises a coaxial strain relief member disposed over the outer coaxial polymer layers at the proximal end of the variable stiffness electrically conductive catheter shaft at the proximal end of the at least one electrically conductive member. The strain relief member is preferably formed of a material and constructed so that the transition in stiffness from the proximal hub to the composite shaft is not abrupt, and can be made of a heat shrink or low durometer elastomeric nylon or Hytrel polymer, such as 25-40 D, for example. The strain relief member is assembled onto the composite shaft by injecting or swabbing an adhesive, such as a UV curable or cyanoacrylate adhesive, over the proximal end of the composite shaft, and by sliding the strain relief member over the proximal end of the at least one electrically conductive member and coaxial polymeric layers. The outer surface of the polymeric coaxial layers may also be surface treated with a plasma or corona etch process to facilitate the adhesion of the strain relief member to the composite shaft.
In another aspect of the invention, the variable stiffness electrically conductive catheter shaft further comprises a connecting hub disposed over the strain relief member and the outer coaxial polymer layers at the proximal end of the variable stiffness electrically conductive catheter shaft at the proximal end of the at least one electrically conductive member for connecting the variable stiffness electrically conductive catheter shaft to a source of electrical current. The proximal hub can be assembled onto the strain relief member and composite shaft by trimming the proximal tip as necessary to square the proximal end of the composite shaft, injecting or swabbing an adhesive, such as a UV curable or cyanoacrylate adhesive, over the proximal end of the strain relief member, sliding the proximal hub over the strain relief member, and allowing the adhesive to cure.
For neurovascular use, the overall length of an electrically conductive member pusher can be, for example, from 100 to 300 cm, with the outer diameter of the distal 25 to 45 cm being less than about 1 French (0.0135 inch), and the outer diameter of the proximal length being less than about 2 French (0.025 inch). For peripheral use, the overall length of the catheter can be, for example, from 100 to 300 cm, with the outer diameter of the distal 25 to 45 cm being less than about 5 French (0.063 inch), and the outer diameter of the proximal 100 cm being less than about 6 French (0.075 inch). For cardiovascular use, the overall length of the catheter can be, for example, from 150 to 175 cm, with the outer diameter of the distal 25 cm being less than about 3 French (0.038 inch), and the outer diameter of the proximal 100 cm being less than about 4 French (0.050 inch).
In a further presently preferred embodiment of the invention, the basic construction of the at least one electrically conductive member can be combined with the invention to provide a variable stiffness electrically conductive catheter shaft. In practice, electrically conductive members used for micro-coil delivery and the like are approximately 0.014 inches in diameter, with the outer buffer comprising a layer of approximately 0.001 to 0.002 thickness of polymer over a thin layer of electrical insulation. In one presently preferred embodiment, the outer buffer can be centerless ground to provide a variable thickness characteristic and the at least one electrically conductive member can be manufactured with a thicker than normal buffer to facilitate grinding of the buffer to provide a desired bending stiffness either with or without additional layers of stiffening polymers over the outer surface of the at least one electrically conductive member.
In still another embodiment of the invention, the reinforcing layer on the outside of the at least one electrically conductive member can consist of longitudinal, angled or circumferential windings of high strength fibers which are bonded to the shaft and can be covered by a smooth outer jacket of heat shrink tubing or the like. By use of such a construction, wide variations in stiffness and other physical parameters can be obtained, further extending the uses to which such devices can be put can be put in therapeutic non-invasive procedures.
In still another presently preferred embodiment, the catheter shaft can be subject to an extrusion process in which at least one polymer is deposited on the exterior of the catheter shaft as it is pulled through extrusion dies. For example, an electrically conductive member can be drawn through a plurality of such dies, each of them depositing a material of a different hardness on the exterior, with the thickness of the deposit being varied by the speed of the at least one electrically conductive member through the die and the temperature of the material being deposited. In this way, tapers to multiple layers can be applied and the overall outside diameter controlled to a desired level. The process can be contained to produce multiple composite shafts in a controlled process, with the shaft cut at desired places to produce individual shafts.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.